Method of making gypsum cement



Dec. 3, 1946. J. P. 0. PETER 1 2,412,170

METHOD OF MAKING GYPSUM CEMENT Filed July 1, 1941 C44 c/A z-p PFODK/CTINVENTOR. u /wv R c. irzv? Patented Dec. 3, 1946 2,412,170 METHOD OFMAKING GYPSUM CEMENT John P. 0. Peter, Westfield, N.

J assignor to American Cyanamid Company, New York, N. Y., a corporationof Maine Application July 1, 1941, Serial No. 400,629

2 Claims.

The present invention relates to a method of drying wet gypsum and isparticularly applicable as a step preparatory to calcination thereof andincludes mixing hot calcined gypsum therewith in a quantity suflicientto reduce the free moisture content of the mixture to a desired point.The mix is then subjected to calcination in the usual manner.

It frequently happens that gypsum, at the time that calcination is totake place, is wet, either by design or accident. This is particularlytrue of a by-product gypsum such as that produced by the reactionbetween phosphate rock and sulfuric acid. While the invention isparticularly adapted for the treatment of by-product gypsum, yet it isto be understood that it is not to be limited thereto.

By-product gypsum, after being filtered from the acid menstruum, isusually washed, thickened and classified and finally centrifuged as dryas possible. Even at this point, however, it may contain from to 20%moisture although usually this figure is approximately 12%.

Experience has shown that if this moist gypsum is then loaded into thecalcining kettles to drive off, first, free moisture and thenthree-quarters of the combined water to produce calcined gypsum orcalcium sulfate hemihydrate, the moist gypsum cakes on the bottom andsides of the kettle, tending not only to burn out or deteriorate thekettles very rapidly, but, unless handled very carefully, will actuallystop the stirring apparatus.

In order to avoid this, such moist gypsum must be added very gradually.As a result, the rate of calcination is retarded. This is illustrated bythe fact that with kettles of the same size, dry gypsum may be filledinto an empty kettle in approximately 45 minutes, whereas 'wet gypsumcontaining, say 12% moisture, requires between one and one-quarter andone and one-half hours to fill one-half a kettle.

Calcination of a moist gypsum also tends to produce a product which isinferior in working qualities and physical properties to ypsum calcinedin the substantial absence of free moisture for the reason that in theformer instance, there is a tendency to over-burn the gypsum, that is,to produce substantial quantities of calcium sulfate anhydrite.undesirable in that plaster or stucco made therefrom does not carry asufficient quantity of water, it takes longer to set and the initialstrength of the product is reduced.

It has been discovered that if a wet gypsum is mixed with hot calcinedgypsum in a ratio suf- This overburned gypsum is.

ficient to reduce the free moisture content of the mixture to a desiredpoint, that this substantially dry mixture may then be loaded into thecalcining kettles at a rate comparable to that used with normally drygypsum and calcination completed without the heretofore existingdifliculties. The accompanying drawing is a flow sheet of the process.

For instance, it has been discovered that if 670 pounds of calcinedgypsum, at a kettle discharge temperature of from 300 to 400 F., ismixed with 1330 pounds of moist gypsum containing substantially 12% offree moisture until the two are thoroughly commingled, the resultingproduct contains not more than 1% free moisture.

Forty tons of good quality calcined gypsum may be prepared from such amixture in approximately ten hours. On the other hand, if moist gypsumcontaining approximately 12% offree water were loaded into the kettleswithout drying, it would require at least eighteen hours to produce thesame quantity of finished product.

Obviously the amount of calcined g psum to be mixed with the wet gypsumwill depend upon the temperature of the calcined product and the amountof free moisture in the wet material. Generally speaking, however, onepart of hot calcined ypsum by weight will be suflicient to reduce thefree moisture content of two parts of wet gypsum. containingapproximately 12% moisture. This moisture figure is that normallyoccurring when wet gypsum has been centrifuged, for unless a special andexpensive type of water removing equipment is used, it will be difiicultto reduce this figure below that point.

The mixing operation does not require expensive or intricate apparatusfor it has been found that an ordinary concrete or plaster mixer or infact, any device capable of thorough mixing of the ingredients to permitescape of excess moisture, is eminently satisfactory. Of course, most ofthe free moisture is removed as such and taken up as water ofcrystallization by the calcined gypsum, although some will actuallyescape as free moisture.

It is to be understood that the invention is not limited to the exactproportions of hot calcined gysum to be added to the wet or moistmaterial as a reasonable leeway in ratio is'to be interpreted as comingwithin the purview of the invention.

While the invention has been shown and described with particularreference to specific embodiments, it is to be understood that it is notto be limited thereto, but is to be construed broadly a41a17o andrestricted solely by the scope of the appended claims.

I claim:

1. A method of producing calcium sulfate hemihydrate from wet gypsumwhich comprises mixing suflicient hot, freshly calcined calcium sulfatehemihydrate with wet gypsum to remove substantially all the freemoisture therein, calcining the resulting substantially dry gypsum attemperatures of 300-400 F. to produce hot calcium sulfate hemihydrate,and returning a portion of said hot calcium sulfate hemihydrate to themixing step.

2. A method of producing calcium sulfate hemihydrate from wet gypsumcontaining about 12% of free moisture which comprises mixing about twoparts by weight of said wet gypsum with one part of hot, freshlycalcined calcium sulfate hemihydrate to remove substantially all thefree moisture in the gypsum, calcinin the resulting substantially drygypsum at temperatures of 300400 F. to produce hot calcium sulfatehemihydrate, and returning a portion of said hot calcium sulfatehemihydrate to the mixing step.

- JOHN P. C. PETER.

